This invention relates to a photographic printing system which can effectively detect and process image information on an original film such as a film negative sheet by a two-dimensional image sensor having a relatively rough picture element density and which can automatically position imaged frames according to the information on feeding direction and size of the original film.
Photographic printing systems need to measure the density of an original film (e.g. a film negative sheet) in order to determine its optimal printing exposure or correction amount. The average density of the film negative is photometrically measured in LATD (Large Area Transmittance Density) in the prior art with photosensors such as photodiodes which are provided near a light path in the photographic printing system. The prior art is defective, however, in that it could not accurately measure exposure or correction amounts because the image detection in the LATD is intended to photometrically measure the average image density of the film negative but not to precisely measure the image density across the whole surface of the film negative. The present applicant has filed applications which have been laid-open under Japanese Laid-open Pat. Nos. 154244/1985, 151631/1985 and 220325/1985 in order to solve the problem. But there arises a dilemma in that if the picture element density of an image sensor is increased, the costs for the image sensor as well as peripheral circuits will be pushed up and the time required for processing is prolonged. For a quick exposure amount operation and so on, the picture element density of the image sensor is preferably relatively rough.
In a photographic printing system, it is further necessary to accurately position a frame of the original film on a photographic frame in order to suitably print images on a sheet of photographic paper. In the case of the film sizes where frames and perforations do not correspond in a 1:1 relationship, like the 110 size and the 126 size, for example, in the case of the 135 size, notches are cut on the sides of a sheet of the original film in the prior art, and then detected by photosensors and then used for positioning. This method, however causes lots of trouble since the notches should be cut with utmost care to have accurate correspondence with the frame positions. There is another positioning method which feeds a sheet of the original film by a predescribed distance, but positional deviations tend to accumulate so as to deteriorate the precision. Still another method proposes that photosensors such as photodiodes are arranged in correspondence to the shapes of frames to position the original film according to the states detected by photosensors and sequence of detection, but this method inconveniently needs a complicated structure and control algorithm. The method is further defective because the slit mask which is mounted in order to enhance resolution tends to lower the sensitivity.
The present applicant proposed the units disclosed in Japanese Laid-Open Pat. Nos. 196740/1985, 185793/1875, etc. in order to solve the problems, but the picture element density of the image sensor required in those units requires a relatively high resolution. This is because unless un-exposed portions on edges or between frames are precisely detected, conveyance of the original film cannot be controlled accurately.
The present applicant proposed still another method (e.g. Japanese Laid-Open Pat. No. 109040/1986) for detecting images on film frames and suspending the feeding by using an image sensor. But this method is applicable only to one directional feeding systems and not to the photographic printing systems which can feed films in vertical as well as lateral directions without complicated mechanical changes; i.e. the attachment position of the image sensor should be mechanically changed depending on the feeding directions of the film. FIG. 1 shows a photographic printing system wherein a film negative 2 can be fed and conveyed in the direction either parallel to or perpendicular to the conveyance direction of a photographic paper 7. This method reduces the types for mounting the photographic paper 7, improves the printing efficiency, and are applicable to film negatives of sizes other than the 126 size which has square frames. There has long been a desire to develop a precise and effective method for detection/suspension of image frames for the above-mentioned type of the photographic printing system which can switch the direction of the film negative feeding.